1. FIELD OF THE INVENTION
The present invention relates in general to a system for sampling gas and, in particular, is directed to a system for sampling a gas containing a reactive particulate solid phase such that essentially all of the solids are removed in such a manner that the gas phase composition is essentially unchanged. Thus, a representative gas sample is obtained for determining its composition by a gas analyzer.
2. DESCRIPTION OF THE RELATED ART
Gas sampling systems are available from several vendors such as E. I. DuPont, etc., for use with gas analyzers. These systems, when designed for dust-laden gases, clean the gas by filtration through a mesh screen or porous media. Where chemical reactions between the gas and particulate matter potentially exist, these systems inadvertently allow these chemical reactions to alter the chemical composition of the gas sample by providing an intimate contact zone. Thus, the gas analysis equipment measures gas concentrations unrepresentative of the bulk gas stream.
Flue gas from fossil fuel fired boilers is one example of this kind of gas-solid mixture. Recent concerns and awareness in our environment have led to new efforts to refine our boiler technology with the removal and/or reduction of air pollutants such as particulates, sulfur oxides (SO.sub.x), and oxides of nitrogen (NO.sub.x).
During the combustion of fossil fuel, various combustion off-gases are produced which contain a variety of contaminants such as sulfur dioxide, sulfur trioxide, and fly ash. U.S. Pat. No. 4,452,765, which is assigned to the assignee of the present invention, discloses a method for removing sulfur oxides from a hot flue gas by introducing an alkali slurry. This patent is hereby incorporated by reference. The present invention finds particular utility in sampling the gas stream at various points in that system to maintain air pollution emission control standards.
Accurate monitoring of the flue gas is required to be sure that methods like this or newly developed ones are effective so that as a minimum they improve the quality of the emission. A representative sample of the gas is necessary for an accurate analysis.
The prior art has recognized some of the problems of analyzing dust-laden gas samples. U.S. Pat. No. 4,485,684 issued to Weber, et al discloses an apparatus for extracting and analyzing dust-laden gas samples. The device employs a stilling chamber tapered downwards in the shape of a horizontal half-funnel in the direction of the flow of the gas. Flanges connect the stilling chamber to a gas sample extraction pipe or sample probe from an exhaust gas line. The gas sample extraction probe of conventional construction extends coaxially in the connecting pipe. A conveying pipe which is connected to a three-way valve acts as a switching valve and connects the gas probe to a conventional filter. From the filter the gas sample goes through a gas feed pump to a gas analyzer. A time control device connected to the three-way valve permits cleaning with compressed air at specific intervals.
A different approach to this problem was used in U.S. Pat. No. 3,106,843, issued to Luxl. This reference discloses an atmosphere sampling probe for gas analyzers to obtain continuous flow of the sample stream. The clogging of the probe during extended periods of operation is prevented by utilizing steam which condenses about the solid particles in the sample stream. Water is supplied to separate the steam by condensing it as well as washing the gas sample of corrosive materials.
Both of these references only address the problem of the filters clogging or plugging with dust. None of these prior art systems recognize the problem that chemical reactions occur between the gas and the particulate material. Nor is the prior art directed to a gas sampling system for sampling a gas containing a reactive solid phase such that the majority of the solids are removed in such a fashion that the gas phase composition is essentially unchanged and is thereby representative of the gas sampled at the initial sampling point.